FIG. 1 shows a conventional transceiver according to the state of the art. The transceiver is provided for data transmission with a multi-tone data signal over a wire.
The transceiver comprises a coding unit for generating a multi-tone signal on the basis of a received data bit stream coming from a data source. FIG. 2 shows as an example a diagram for a single tone QAM coding scheme. A data bit sequence of e.g. 2 bits is transformed into a sine carrier with a predetermined amplitude and phase.
The transceiver comprises an inverse fast fourier transformation unit (IFFT) to produce a discrete time domain signal which is converted by a transmission signal path of the transceiver into a continuous time domain signal. The signal transmission path of the transceiver comprises a digital filter, a digital analogue converter, an analogue filter and a driving circuit. The digital and the analogue filter are provided for forming the transmission signal. The driving circuit amplifies the continuous time domain signal which is transmitted over a telephone line to a customer's device.
The transceiver further comprises a reception signal path having an analogue filter, an analogue/digital converter and a digital filter. The analogue filter is provided for limiting the frequency band of the received analogue signal. The digital filter is provided for forming the reception signal. A fast fourier transformation unit is provided for transforming the discrete time domain signal into the frequency domain. A decoding unit derives a bit combination from the amplitude and phase of the different sine carriers and supplies a data bit stream to a data sink. An echo cancellation unit compensates echo signals.
A multi-carrier transmission is composed of a number of independent signals. By using discrete multi-tone transmission (DMT), a transmission data symbol is generated by adding several independent sine carrier signals. FIG. 3 shows a diagram with several sine carriers having different amplitudes and different frequencies. A multi-tone data signal comprises a plurality of tone signals which normally have frequencies which are equidistant to each other. When using DMT, the amplitude and phase of the sinus carriers is kept constant for a symbol time period
      TS    symbol    =            1              Δ        ⁢                                  ⁢        f              .  
As can be seen from FIG. 3, since the data transmission symbol is generated by superimposing different sine carriers, signal peaks can occur.
FIG. 4 shows a transmission time signal of a conventional transceiver with some signal peaks. Signal peaks occur with a probability having a Gaussian distribution as shown in FIG. 5. The signal peaks can exceed the output capabilities of the transceiver, since the driving circuit comes into saturation and the peaks of the transmitted signals are cut off. Saturation causes the transmitted signal to lose a significant amount of information which might be corrected on the receiving side. It is important to reduce the signal peaks of the transmission signal in order to maintain the integrity of the transmitted signal. Accordingly, it is desired to reduce the crest factor of the transmission signal, i.e. the peak to average power ratio of the transmission signal.